The disclosure of U.S. Pat. No. 5,135,309 of the same assignee as the present invention, which issued Aug. 4, 1992 is incorporated herein by reference. That patent discloses measurement processes and apparatus in which several fringe patterns are projected onto an object for highly precise measurement within a large region of certainty. Phase measurement values from several projection arrangements are evaluated such that first phase measurement values arise relating to a first spatial periodicity with a large contour surface distance (long periodicity) within the depth of the measurement volume and second phase measurement values arise relating to a second spatial periodicity with small contour surface distance (short periodicity). A contour surface here means a surface of the same phase position.
The phase measurement values relating to the long periodicity make possible a determination of the surface coordinates that is unambiguous, though inexact, within the measurement volume. The phase measurement values relating to the short periodicity on the other hand make possible an ambiguous determination of the surface coordinates, from which a value can be selected unambiguously with the aid of the long periodicity.
Reference phase values obtained during a calibration measurement are subtracted from the calculated phase measurement values for the measurement object, in order to correct for errors due to imperfections in the mechanical and/or optical construction. However, only reference phase values are considered that were obtained by measurement on a single reference surface. These do not supply information of any kind about the variation of the errors within the measurement volume, so that in spite of the calibration measurement a nearly perfect construction is required.
Calibration processes for fringe projection processes or projection moire processes are known from DE-OS (German Offenlegunschrift) 3,813,692 (Kwarta et al.) and DE-PS (German Granted Patent) 4,011,780, in which numerous calibration measurements in mutually parallel planes are carried out. However, only patterns of a single period are worked with, so that unambiguous measurement values can be achieved with these measurement processes only within a single period. No teachings of any kind can be gleaned relating to measurement processes and apparatus with which signals of different periods are used for calculating the object topography. Moreover this calibration process is very time-consuming, because calibration measurements in very many parallel planes, e.g., in 32 planes according to DE-OS 3,813,692, have to be carried out.